This invention relates to traveling wave devices, and more particularly, to stable operating regimes for high power traveling wave devices.
The free-electron laser (FEL) and the traveling-wave tube (TWT) are both traveling wave devices (TWDs) in which a traveling radio frequency (RF) wave is in synchronism with an electron beam and exponentially extracts power from the electron beam. In a FEL, the RF wave travels faster than the electrons, but the synchronism is established either by wiggling the electrons (in a standard FEL) or by wiggling the RF (in an axial free-electron laser). In a TWT, the RF wave is slowed down in a "slow-wave structure," and no wiggling is required to establish synchronism.
Free-electron lasers (FELs) have demonstrated both high beam-to-radio-frequency (RF) power extraction efficiences (.about.30%) and high output power (on the order of gigawatts), and have been considered as candidates to drive high-frequency advanced accelerators like those proposed for linear colliders. However, poor phase stability has been measured for FELs. Typical accelerator applications require RF phase stability on the order of 5.degree. of phase, and advanced accelerator applications, such as bunch compression and short-wavelength FELs, require stability to 1.degree. or less. At low frequencies, klystrons can meet these requirements, which is one reason they are used so extensively for driving accelerators.
Phase noise in microwave FELs arises from fluctuations in tube voltage, current, confining magnetic field strength, and other tube parameters. Typically, the largest effect is from voltage fluctuations. Electron beams for practical FELs used as RF sources will have diode voltages of 1/2 to 1 MV with voltage stabilities on the order of 1/4%. Measured and simulated FEL phase stability to date, which has all been done at high frequencies, has been on the order of 20.degree. to 40.degree. shift per percent voltage fluctuation. This level of phase stability does not satisfy advanced accelerator requirements.
The magnitude of the phase dependency on the beam voltage is easily understood by considering how the output phase is related to the transit time of the electron beam as it travels through the microwave device. In addition, for an FEL, the growing mode's phase velocity depends on several other factors that are dependent on the beam voltage, such as current, plasma frequency, and interaction strength between the electrons and the RF field.
It has been shown for cyclotron autoresonance maser (CARM) amplifiers that it is possible to introduce a correlation in the transverse motion of the electrons with respect to the beam voltage by using a bifilar helical wiggler. The interaction strength is then a function of beam voltage, and it is possible to design the device such that phase variations due to changes in the beam's transit time effectively cancel variations in the phase due to changes in the interaction strength as the beam voltage fluctuates. The proper correlation has been analyzed for the case of negligible space charge forces for a CARM amplifier. This phenomena was named autophase stability.
It is not always easy or convenient to provide a correlation of the interaction strength that will provide autophase stability, particularly for non-CARM interactions. For example, the interaction strength of most TWDs using mildly relativistic electron beams with constant perveance guns has only a weak dependence on the beam voltage. However, in accordance with the present invention, it is relatively easy to generate a correlation with the space-charge wave of the beam that will provide autophase stability simply by detuning the nominal beam energy away from synchronism for interaction strengths that are even independent of the beam voltage. For typical interaction strength dependencies on the beam voltage, low-energy TWDs can be made phase stable, both in the low- and high-gain regimes.
Practical FEL RF sources for linear collider applications need to produce at least several hundreds of megawatts of RF power. In order to accomplish this, the electron beam needs to contain several kiloamperes of current and must be annular to prevent exceeding the space-charge limiting current. In another aspect of the present invention, an off-axis or annular electron beam for a Raman-region FEL introduces the ability to control the reduced plasma frequency of the beam by decreasing the beam wall spacing, thereby shunting the beam's space-charge field to the beam pipe wall and increasing the so-called "plasma reduction factor."
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.